Addition Of Silyl Enol Ethers Research Articles

Gold(I)‐Catalyzed Cyclizations of Silyl Enol Ethers: Application to the Synthesis of (+)‐Lycopladine A

In control: Gold(I)-catalyzed intramolecular addition of silyl enol ethers to alkynes and allenes allows for the diastereoselective synthesis of cyclopentenes with control of the position of the double bond. The utility of these reactions is demonstrated by an efficient total synthesis of (+)-lycopladine A that takes advantage of the orthogonal reactivity of AuI and Pd0 towards unsaturated iodides (see scheme).

Copper(I)-Fesulphos Lewis Acid Catalysts for Enantioselective Mannich-Type Reaction of N-Sulfonyl Imines

[reaction: see text] Copper(I) complexes of Fesulphos ligands are efficient chiral Lewis acid catalysts in the Mannich-type addition of silyl enol ethers of ketones, esters, and thioesters to N-(2-thienyl)sulfonyl aldimines. The corresponding optically active beta-amino carbonyl derivatives were obtained in good yields (58-91%) and with moderate to good enantioselectivity (61-93% ee). Removal of the N-activating group was achieved under mild conditions by simple treatment of the products with Mg in methanol.

Intramolecular Additions of Various π-Nucleophiles to Chemoselectively Activated Amides and Application to the Synthesis of (±)-Tashiromine

[reaction: see text] Vilsmeier-Haack type cyclizations proved to be particularly efficient for generating parts of the polycyclic cores of many alkaloids, although only monocyclizations have so far been reported. With the goal of rapidly and efficiently constructing polycyclic alkaloids, we decided to exploit the Vilsmeier-Haack reaction by utilizing iminium ions successively generated and trapped with tethered nucleophiles. To develop such a strategy, we had to set the first cyclization. This constitutes a great challenge in itself because amide activation conditions are usually not compatible with tethered nucleophiles, except for indoles and aromatic rings which have already been reported. This paper describes the comprehensive study of intramolecular addition of silyl enol ethers, allylsilanes, and enamines to chemoselectively activated formamides, aliphatic amides, and lactams. Good to excellent yields were obtained for the 5-exo, 6-exo, and 6-endo modes of cyclization. Moreover, we demonstrated that the species in solution after the cyclization are iminium ions. This is highly encouraging for the development of bis-cyclization strategies. An expeditious total synthesis of (+/-)-tashiromine is also reported.

Lewis Base Activation of Lewis Acids. Catalytic Enantioselective Addition of Silyl Enol Ethers of Achiral Methyl Ketones to Aldehydes.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Lewis base activation of Lewis acids. Catalytic enantioselective addition of silyl enol ethers of achiral methyl ketones to aldehydes.

A highly enantioselective addition of silyl enol ethers derived from simple methyl ketones is described. The catalyst system of silicon tetrachloride activated by a chiral bisphosphoramide (R,R)-7 effectively promotes the addition of a variety of unsubstituted silyl enol ethers to aromatic, olefinic, and heteroaromatic aldehydes in excellent yield. [reaction: see text]

Lewis acid promoted, O-selective, nucleophilic addition of silyl enol ethers to NdoublebondO bonds.

Not N-adduct but O-adduct is formed in the Lewis acid catalyzed nucleophilic addition of silyl enol ethers 2 to the NO bond of nitrosobenzene (1). Various Lewis acids (e.g. alkylsilyl triflates) efficiently catalyze the formation of the aminooxy ketone (O-adduct 3) rather than of the hydroxyamino ketone (N-adduct 4), which is the product of the uncatalyzed reaction.

Zinc tetrafluoroborate catalyzed Mannich-type reaction of aldimines and silyl enol ethers in aqueous medium

The use of zinc tetrafluoroborate as a catalyst in Mannich-type addition of silyl enol ethers to aldimines in aqueous THF provides β-amino ketones or β-amino esters in high yields. A one-pot reaction of an aldehyde, amine and silyl enolate also works well.

Lewis acid-mediated intramolecular addition of silyl enol ethers to internal unactivated alkynes

The EtAlCl2-mediated intramolecular addition of silyl enol ethers to both terminal and internal unactivated alkynes, bearing alkyl and phenyl susbstituents at the alkyne moiety, gave mono- and bicyclic β,γ-unsaturated ketones in good to excellent yields. On the other hand, the silyloxy-substituted cyclic vinylsilanes were obtained in moderate to high yields when the reactions were catalyzed by B(C6F5)3 in the presence of triethylsilane. All the reactions proceeded via endo-fashion exclusively. The mechanisms of these regiospecific Lewis acid-assisted carbocyclizations are proposed.Key words: Lewis acid, silyl enol ethers, carbocyclization, alkynes.

Lewis acid-mediated intramolecular addition of silyl enol ethers to internal unactivated alkynes

Lewis acid-mediated intramolecular addition of silyl enol ethers to internal unactivated alkynes

Dichloro[TADDOLato(2−)‐ O , O ′]titanium/Dichlorobis[1‐methylethoxy]titanium‐Mediated, Highly Diastereo‐ and Enantioselective Additions of Silyl Enol Ethers to Nitro Olefins and [3+2] Cycloadditions of Primary Adducts to Acetylenes

The diastereoselective, Ti-Lewis-acid-mediated, low-temperature addition of silyl enol ethers to 1-aryl-2-nitroethenes (Scheme 1) occurs enantioselectively with dichloro[TADDOLato(2−)-O,O′]titanium 3 (TADDOL=α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanol) (Scheme 2). At least 3 equiv. of Lewis acid are required for high conversions (yields). However, the chiral Lewis acid 3 can be `diluted' with the achiral Cl2Ti(OCHMe2)2 analog (ratio 1 : 2.5), with hardly any loss of enantioselectivity! Both, the primary (4+2) cycloadducts (B, 9) and the γ-nitro ketones (A, 1a – h, 5, 7), formed by hydrolysis, can be isolated in good yields and with high configurational purities (Schemes 3 and 4, and Table 1). The relative and absolute configurations (2S,1′R) of the products 1 from cyclohexanone silyl enol ether and 1-aryl(including 1-heteroaryl)-2-nitroethenes (obtained with (R,R)-TADDOLate) are assigned by NMR spectroscopy, and optical comparison and correlation with literature data, as well as by anomalous-dispersion X-ray crystal-structure determination (nitro ketone 1c; Fig.). The nitro ketone 7 from cyclohex-2-enone and 4-methoxy-β-nitrostyrene was cyclized (via a silyl nitronate C; Scheme 5) to the nitroso acetal 8, and one of the bicyclic nitronate primary adducts 9 underwent a [3+2] cycloaddition to phenylacetylene and to ethyl 2-butynoate to give, after a ring-contracting rearrangement, tricyclic aziridine derivatives with five consecutive stereocenters (10, 11; Scheme 5 and Table 2), in enantiomerically pure form. With an aliphatic nitro olefin, the Ti-TADDOLate-mediated reaction with (silyloxy)cyclohexene led to a moderate yield, but the product 4 was isolated in a high configurational purity.

The first addition of silyl enol ethers to internal unactivated alkynes

The EtAlCl 2-mediated intramolecular addition of silyl enol ethers to both terminal and internal unactivated alkynes, bearing alkyl and phenyl susbstituents at the alkyne moiety, proceeded exclusively in the endo-fashion to give mono- and bicyclic β,γ-unsaturated ketones in good to excellent yields. The mechanism of this regiospecific Lewis acid-assisted carbocyclization is proposed.

Asymmetric catalysis with chiral Lewis bases

Abstract

ChemInform Abstract: Diastereoselective Addition of Silyl Enol Ether to Chiral Imines and 1,3‐Oxazolidines Using a Lewis Acid.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Diastereoselective Addition of Silyl Enol Ether to Chiral Imines and 1,3-Oxazolidines Using a Lewis Acid

The Lewis acid promoted reaction of silyl enol ether to chiral imines and 1,3-oxazolidines derived from (R)-phenylglycinol afforded with good diastereoselectivity the (R,R)-β-amino ester derivatives from the imines, and the (R,S)-β-amino ester derivatives from the 1,3-oxazolidines.

Addition of silyl enol ethers and silyl ketene acetals to ferrocenylmethyl ethers. Synthesis of precursors to chiral bridged ferrocenophanes

Ferrocenylmethyl methyl ethers react cleanly with silyl enol ethers and silyl ketene acetals promoted by BF 3OEt 2 to give β-ferrocenylcarbonyl adducts in excellent yields. Silyl enol ethers react with up to 3: 1 selectivity for the anti diastereoisomer. Meso 1,1′-bis(α-hydroxybenzyl)ferrocene reacts with methanol and acetic acid to give exclusively meso 1,1′-bis(α-methoxybenzyl)ferrocene. Both this and the corresponding racemic diastereoisomer may be separated by recrystallisations of an initially 1:1 mixture. Racemic and meso 1,1′-bis(α-methoxybenzyl)ferrocenes react with 1-ethoxy-1- (trimethylsilyloxy)ethene promoted by BF 3.OEt 2 to give racemic and meso β-ferrocene adducts with retention of relative configuration as determined by an X-ray crystal structure analysis of the corresponding meso 1,1′-diacid obtained on hydrolysis

Photochemical Michael reaction of silyl enol ethers with 2′-nitro- and 2′,2′-dicyanostyrenes

Photolysis of silyl enol ethers with 2′-nitro- and 2′,2′-dicyanostyrene acceptors yields the corresponding Michael adducts in moderate yield and diastereoselectivity. The desilylation of the silyl enol ethers to the corresponding ketones and cis-trans isomerization of the olefinic acceptors competes with the photochemical Michael addition. Under similar conditions, α,β-unsaturated carbonyl compounds give only cis-trans isomerization. A mechanism involving the regiospecific addition of silyl enol ether to the π-π* excited Michael acceptor to form a zwitterionic intermediate is proposed.

Addition of silyl enol ethers to imidazoles and thiazoles in the presence of alkyl chloroformate

Various kinds of silyl enol ethers and ketene silyl acetals react with imidazole, thiazole and their benzo-derivatives in the presence of an alkyl chloroformate to give 2-substituted imidazolines and thiazolines in good yields.

ChemInform Abstract: Chemoselectivity in the Michael Addition of Silyl Enol Ethers in Lithium Perchlorate‐Diethyl Ether Medium. Evidence for Facile Silyl Group Transfer to Michael Acceptors.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Chemoselectivity in the Michael Addition of Silyl Enol Ethers in Lithium Perchlorate-Diethyl Ether Medium. Evidence for Facile Silyl Group Transfer to Michael Acceptors

Chemoselectivity in the Michael Addition of Silyl Enol Ethers in Lithium Perchlorate-Diethyl Ether Medium. Evidence for Facile Silyl Group Transfer to Michael Acceptors

Reagent control of cram-type selectivity in the mukaiyama aldol catalysed by supersilylating agents

In the addition of silyl enol ethers 8 to α-asymmetric aldehydes 1 and 2, catalysed by ‘supersilylating agents’ R3SiB(OTf)4, †; the level of Cram-type selectivity correlates with the steric bulk of the silyl group; use of the triisopropylsilyl enol ether 8d results in unprecedented levels of 1, 2-asymmetric induction.